This invention relates to a semiconductor apparatus having at least a Zener diode.
It is generally known to constitute a Zener diode by forming two semiconductor regions of different conductivity types in contact with each other within a semiconductor layer. FIGS. 1A and 1B illustrate a plan pattern and a cross-sectional structure of a prior art semiconductor apparatus having a Zener diode of this kind. This semiconductor apparatus includes a p-type silicon substrate 2, a p.sup.+ -type isolated region 4 which serves to form an n.sup.- -type island region 6 by isolating part of an n.sup.- -type epitaxial layer formed on the substrate 2 from other regions, an n.sup.+ -type buried layer 8 formed in the boundary region between the island region 6 and the substrate 2, a p.sup.+ -type anode region 10 formed in the surface area of the island region 6, and an n.sup.+ -type cathode region 12 formed in the surface areas of the island region 6 and anode region 10. Anode and cathode contact layers 14 and 16 are so formed as to come into electrical contact with the anode and cathode regions 10 and 12.
In this semiconductor apparatus, the pn junction area between the anode region 10 and the cathode region 12 forms a Zener diode. When a positive surge voltage is applied to the cathode contact layer 16, a fairly large surge current flows therefrom to the anode contact layer 14. In this case, since almost all of this surge current flows through the pn junction area between the anode and cathode regions 10 and 12, there is a considerable possibility such that the pn junction will be destroyed. Therefore, conventionally, the area of the pn junction between the anode and cathode regions 10 and 12 is enlarged, or a resistor is connected in series to this Zener diode in order to increase the withstanding surge voltage of the Zener diode. However, in the ordinary operational mode, an increase in the area of the pn junction causes the density of the signal current passing through the pn junction to be reduced, so that the influence of Zener noise is not negligible. On the other hand, the connection of a resistor to the Zener diode causes the operating resistance of the Zener diode to be increased, so that although the signal will be attenuated the operational characteristics of the Zener diode are deteriorated.